The NMR experiment is based on observing the spin system response invoked by a radiofrequency (“RF”) pulse. The large difference between levels of transmit (“Tx”) and receive (“Rx”) NMR signals requires a high isolation between Tx and Rx channels. In modern NMR scanners, isolation is usually achieved by separating these processes in time (i.e., by performing acquisition after pulsed excitation) or more rarely in frequency (i.e., by using different frequencies for Tx or Rx). In early NMR experiments, an alternative approach was used whereby Tx and Rx were performed simultaneously at the same frequency. This method can be generally referred to as simultaneous transmit and receive (“STAR”).
The STAR approach can have a number of advantages for MRI. As one advantage, the distribution of RF power in time results in a decrease of the RF peak-power requirement to just a few percents of the RF peak power required in a conventional pulsed Fourier transformation (“FT”) mode. As another advantage, the absence of a delay between spin excitation and acquisition allows signal detection from most spins, including those with ultra-short spin-spin relaxation times (T2 and T2*). As still another advantage, the excitation in STAR can be done efficiently in the frequency bandwidth of interest without wasting energy outside of this bandwidth, unlike broadband pulsed excitation or gapped excitation, which create unused sidebands. This tailored excitation can considerably decrease the specific absorption rate (“SAR”), which is a limiting factor for high field magnetic resonance imaging (“MRI”), or in the imaging of low gamma nuclei (i.e., nuclei with a small gyromagnetic ratio).
Due to mutual coupling between the Tx and Rx ports in an RF coil, some of the transmit RF signal leaks into the receiver. Without optimal Tx-Rx isolation, the leakage signal level exceeds the RF input power maximum of the first, low noise pre-amplifier (“LNA”) at the receiver. In this case, it is difficult to extract the desired NMR signal from the received signal. Therefore, in a STAR system, the mutually coupled leakage signal must be cancelled out or at least decreased to below the threshold of the first LNA by using additional passive devices, active devices, or both.
Recently, a few published works have demonstrated a “proof-of-principle” for the STAR approach in the field of MRI. In one example described by D. Idiyatullin, et al., in “Continuous SWIFT,” J Magn Reson., 2012; 220(0):26-31, Tx-Rx isolation was increased to a level compatible with successful imaging by using a standard quadrature hybrid with a quadrature driven coil and a slightly detuned resonance condition for internal compensation of the leakage signal with reflected power. In another example described by A. C. Ozen, et al., in “Active Decoupling of RF Coils: Application to 3D MRI with Concurrent Excitation and Acquisition,” Proc. 23rd scientific meeting, ISMRM, Toronto, Canada, p. 750 (2015), an additional transmit coil was used to decouple Tx and Rx ports.
While high isolation can be achieved between transmit and receive in a quadrature driven coil, slight load changes within the coil, such as those that occur with subject movement, can quickly and significantly degrade the tune, match, and STAR isolation in the coil. Unfortunately, both of the methods mentioned above are highly sensitive to the RF coil's loading conditions, which make their use impractical for in vivo MRI.
Because Tx-Rx isolation achieved by geometrically decoupled transmit from receive fields in an RF coil is not stable enough to accommodate variable in vivo load conditions, a STAR system with a load-insensitive design must be added between the coil and a receiver chain. Thus, there remains a need for an RF system capable of implementing simultaneous transmit and receive under variable loading conditions.